Modern automotive security systems may require a very fast reaction time. This is particularly true for AIRBAG systems wherein the inflation of the AIRBAG is triggered by a squib module. It has further been found that power transistors driving the squib module should react very fast so that a well-timed inflation of the AIRBAG can be achieved. Furthermore it was found out that power-MOSFETs (MOSFET=metal-oxide-semiconductor field-effect transistor) are well-suited for driving the squib module of an AIRBAG.
A power-MOSFET may be typically driven by an appropriate gate driver circuit. In power-MOSFET driver circuits, the most important design parameters to be fulfilled are the transient response to load variations and the AC stability issue. Furthermore, it should be noted that in actual AIRBAG applications we face growing driving current capabilities and faster slew rates of load current.
An actual AIRBAG application will subsequently be described with reference to FIGS. 17 and 18. FIG. 17 shows a block schematic diagram of a conventional AIRBAG driver circuit, and FIG. 18 shows a graphical representation of a maximum current slew rate and a maximum current as a function of time for the AIRBAG driver circuit of FIG. 17.
The schematic block diagram of FIG. 17 shows an AIRBAG application system. The airbag driver circuit of FIG. 17 is designated in its entirety with 2100. An input voltage VER is available at the input of the AIRBAG application system 2100. Input voltage VER is usually obtained by a boost converter in series with the battery voltage in order to get a voltage much higher than the battery voltage for AIRBAG safety purposes. The input energy provided by the battery and/or the boost converter (both not shown here) is then stored in a big “reservoir” and capacitor CER in the order of tenths of mF. The capacitor CER is followed in series by an anti-reverse power diode 2109. The input energy stored in the reservoir capacitor CER can be “shorted” to the load (e.g. an AIRBAG SQUIB module) via a power-MOS-switch 2102.
Special requirements with respect to the shown AIRBAG application system is the capability to sustain very fast variations of load current in order to provide current to the AIRBAG squib modules 2107, 2108 exactly they require this current. It should be noted here that the AIRBAG squib modules 2107, 2108 request current exactly when the AIRBAG security system managed by the central processing unit (CPU) 2105 detects that the firing condition is fulfilled and the CPU has already switched the system from the normal stand-by mode to the firing mode. In other words, the CPU 2105 may preferably switch from stand-by mode to firing mode before the squib modules are triggered.
The block named “Power-MOS-switch” 2102 plus the block “Power Control System” 2103 has seen a new evolution during the last recent years, starting from a “pure” switch which simply shorts the input voltage VER to the output voltage VOUT. Also, instead of a simple power-MOS-switch driven directly by the CPU, a voltage regulator concept using the power control system 2103 has been shown. The power control system 2103 is responsible for stabilizing a gate voltage provided to the MOS-switch 2102 in order to give the AIRBAG squib modules 2107, 2108 an output voltage VOUT much lower than the input voltage VER. The necessity to use a lower output voltage VOUT comes from a technological change of the voltage class of the AIRBAG squib modules 2107, 2108.
It should be noted that the AIRBAG driver circuit of FIG. 17 further comprises an output capacitor 2106, a current source 2101 connected between the input and the output of the power-MOS-switch 2102 and a diagnosis circuit 2104. Furthermore, at the input of the power-MOS-switch 2102 an additional diode 2110 and an additional buffer capacitor 2111 may be connected in the shown way.
Furthermore, it should be noted that the firing of the AIRBAG in an automobile typically requires a pair of field-effect transistors (FETs) in order to let the current flow through the series of them. Typically, one field-effect transistor of the pair of field-effect transistors is placed at each side of the squib. In order to decrease the area of the field-effect transistors (FETs) on silicon (i.e. to limit the amount of energy they have to dissipate), and even more in order to avoid undesired and very dangerous explosion commands to the squib, a solution with an external additional power element added has already been proposed in US patent application U.S. 2004/0108698A1.
In FIG. 18, a first curve 2210 describes a maximum current slew rate dISD/dt as a function of time, wherein the current ISD is defined in FIG. 17. A second curve 2220 describes the maximum available current ISD as a function of time. For the circuit 2100 of FIG. 17, in operation mode 3 (firing mode) the maximum current slew rate dISD/dt (1.5 A/μs) is reached only after a minimum of 8 μs. The complete timing is specified in the diagram of FIG. 18.
FIG. 19 shows a prior-art solution according to U.S. 2004/0108698A1. In other words, FIG. 19 shows a block schematic diagram of a prior-art AIRBAG driver circuit. The circuit of FIG. 19 is designated in its entirety with 2300. With reference to FIG. 19, the firing circuit 2300 is connected between a supply voltage VSUP and a reference potential GND (also designated as ground GND). A supply circuit or a boost regulator 2301 may be used to provide the supply voltage VSUP. Charge-pumps are used to boost the voltage for firing circuits above the voltage supplied by the vehicle battery (not shown here). A pre-charged capacitor 2302 can be used to supply power to the electronics if the vehicle battery becomes disconnected or damaged during a crash event. For that reason, the pre-charged capacitor 2302 has to be really huge (order of magnitude of some mF). Both a high-side switch 2305 and a low-side switch 2306 are closed during the firing pulse, when the proper command SA on their gates is generated by a microprocessor. In this way, a certain amount of current (typically few amperes of current) can flow through the series connection (consisting of field-effect transistor 2304, high-side switch 2305, squib 2307 and low-side switch 2306) during a small period of time (few milliseconds), thus producing explosions.
It should further be noted that the widespread drive circuit used to explode the squib in AIRBAG deployment systems has a series of a high-side switch to the firing cap and of a low-side switch on the other side, as, for example, described in U.S. Pat. No. 5,631,834 and U.S. Pat. No. 5,135,254.
Furthermore, it should be noted that some circuits intended to define and limit the amount of current (flowing through the series connection) have already been proposed and studied. Some possible solutions are shown in U.S. Pat. No. 5,734,317A and U.S. Pat. No. 5,309,030. However, in the solutions of the prior art the silicon area of the driver transistors 2305, 2306 and thus so far their thermal capacity has been increased so that the driver transistors 2305, 2306 can absorb a high amount of energy. Thus, the problem of a high cost by integrating a large silicon area of the driver transistors using a complex wafer manufacturing technology arises unless the special solution of U.S. 2004/0108698 A1 is adopted.
It should further be noted that if both transistors used for firing are inadvertently activated simultaneously due to some chip-internal fault, this leads to a faulty firing with the consequence that an airbag unfolds without the relative external reason. This can lead to serious accidents. With the solution proposed in U.S. 2004/0108698 A1, there is no need for external auxiliary mechanical closable switches to face this safety problem, because the introduction of another power element 2304 connected in series with the high-side switch 2304 is adopted. If the additional power element 2304 is normally switched off, an unwanted explosion can be avoided. In the shown configuration, the gate voltage of the external N-type channel field-effect transistor (FET) 2304 is generated by a constant voltage source 2303. The output (the source terminal of the additional power element 2304) is driven in a source-follower configuration, wherein the source voltage is determined directly by the value of the gate voltage.
However, in normal operation mode different from the firing mode, i.e. with the gate of the N-type channel field-effect transistor (FET) 2304 not fed but shorted to ground, there is no way available for easily monitoring the squib driver circuitry because no output voltage is guaranteed to the squib driver circuitry 2305, 2306 for diagnostic purposes. Moreover, with the configuration presented in U.S. 2004/0108698 A1 there is no easy way available for monitoring the behavior of the additional power element 2304 itself. Accordingly, there is still a possibility that a malfunction is not detected.
From the above discussion it can be concluded that the prior-art solutions may still comprise severe drawbacks and bring along serious design problems or even security risks. In particular, prior-art power switches for AIRBAG applications are potentially unstable. Furthermore, it may be difficult to perform a complete system diagnosis using a prior-art AIRBAG driver. Besides, the power switches in conventional systems may be typically rather slow due to the introduction of stabilizing networks.